1. Field of the Invention
This invention relates to a process for the preparation of a radiographic intensifying screen.
2. Description of the Prior Art
In a variety of radiography such as medical radiography for diagnosis and industrial radiography for nondistructive inspection, a radiographic intensifying screen is generally employed in close contact with one or both surfaces of an X-ray film for enhancing the speed of the radiographic system. The radiographic intensifying screen comprises a support and a phosphor layer provided thereon. A transparent film is generally provided on the free surface of the phosphor layer to keep the phosphor layer from chemical and physical deterioration.
The phosphor layer comprises a binder and a phosphor dispersed therein. The phosphor is in the form of small particles, and emits light of high luminance when excited with a radiation such as X-rays. The phosphor particles emit light of high luminance in proportion to the dose of radiation supplyed through an object. The X-ray film positioned in close contact with the intensifying screen is exposed to the light emitted by the phosphor, in addition to direct exposure to the radiation supplyed through the object. Accordingly, the X-ray film can be exposed sufficiently to form the radiation image of the object, even if the radiation is applied to the object at a relatively small dose.
As to the radiographic intensifying screen having the principle structure as described above, it is desired that the screen shows a high radiographic speed, as well as provides an image of high quality (sharpness and graininess).
In the conventional process for the preparation of a radiographic intensifying screen, a phosphor layer of the intensifying screen is formed by dispersing phosphor particles in a binder using an appropriate solvent to give a coating dispersion, applying the coating dispersion onto a support, and drying the applied coating dispersion thereonto.
In the above-mentioned procedure for the preparation of the coating dispersion, however, there are such problems that it is considerably difficult to prepare a homogeneous dispersion of the phosphor particles, and that the phosphor particles are apt to sediment independently or in the agglomerated form with the lapse of time, and as a result, cakes of the agglomerated phosphor particles produce. The so produced cakes of the agglomerated phosphor particles make it difficult to apply the coating dispersion uniformly onto the support. Further, the cakes of the agglomerated phosphor particles deteriorates the radiation speed of the resultant radiographic intensifying screen and the quality of image provided thereby. Especially, the above-mentioned tendency of sedimentation of phosphor particles is noted in the case of using phosphor particles having a high specific gravity.
For improvement of the dispersibility of the phosphor particles in a coating dispersion, a method of adding to the coating dispersion a dispersing agent such as phthalic acid, stearic acid, caproic acid or a hydrophobic surface active agent is proposed, and employed in practice.